1. Field of the Invention
The present invention relates to a method of and an apparatus for liquid-phase epitaxial growth to obtain thin epitaxial layers grown on a plurality of thin plate-like substrates.
2. Description of the Prior Art
The liquid-phase epitaxial growth method is a process for precipitating and growing the solute in a super-saturated solution on a substrate which is brought into contact with the supersaturated solution. For industrial purposes, this method is used mainly for the production of light emitting diodes (LEDs), in which instance molten metals are used as a solvent to form a thin semiconductor layer with a desired impurity concentration on a semiconductor substrate. There are known several techniques used to carry out the liquid-phase epitaxial growth method for producing LEDs.
FIG. 7 of the accompanying drawings shows one such known technique generally known as a tipping method in which a solution S used for the liquid-phase epitaxial growth is held at a lower portion of a reaction tube 2 with a space H defined at an upper portion of the reaction tube 2. A substrate holder 4 is disposed in the space H and a semiconductor substrate W is held horizontally on an upper surface of the substrate holder 4. The reaction tube 2 is turned through an angle of 180 degrees around its axis so that an upper surface of the semiconductor substrate W is directed downward and immersed in the solution S, as shown in FIG. 8. Due to contact between the solution S and the semiconductor substrate W, a thin epitaxial layer is grown on the semiconductor substrate W. However, since the semiconductor substrate W while being in contact with the solution is held parallel to the surface of the solution, only a limited number of semiconductor substrates W can be received at one time in the reaction chamber. This method is, therefore, not suited for mass production.
FIGS. 9 and 10 show two other known methods in which substrates W are held vertically within a substrate holder 6 and a solution S falls down by gravity along the surface of the substrates W. In the method shown in FIG. 9, a horizontal partition plate 8 is pulled outward to allow the solution S to fall down along the surface of the substrates W. Upon expiration of a predetermined period of time, the solution S is removed from the substrate holder 6. The method shown in FIG. 10 is a modification of the method shown in FIG. 9 and includes an upper horizontal partition plate 8a which is moved forward to allow the solution S to fall down along outside surface of the substrates W. When a predetermined period of time is expired, a lower horizontal partition plate 8b is forced forward to allow the solution S to fall down into a solution tank 10, thereby separating the substrates W and the solution S. These known methods are able to process an increased number of substrates W at one time within the substrate holder 6. However, due to the difference in specific gravity between an upper portion and a lower portion of the solution S, the solute tends to exhaust at the lower portion of the solution S so that the thickness and impurity concentration of epitaxial growth layers vary between an upper portion and a lower portion of each individual substrate W. In addition, the solution S dropped from the substrates W cannot be used again.